1. Field of the Invention
This invention relates to a double-walled container for cryogenic liquid of the type having spaced-apart inner and outer wall members enclosing a sealed insulation space, and to a method of fabricating same.
2. Description of the Prior Art
For storage and dispensing of cryogenic liquids, double-walled containers are commonly employed which comprise spaced-apart inner and outer wall members enclosing a sealed insulation space. The insulation space of such containers frequently contains a low conductivity thermal insulation material to enhance the insulation quality of the insulation space.
In containers of the above-described type, the insulation space is advantageously evacuated to provide and maintain at least partial vacuum conditions in the space so as to reduce heat leak via gaseous conduction and convection from the outer wall member at ambient temperature to the inner wall member adjacent the cryogenic liquid. For such reason, sorbent materials such as physical adsorbent materials and getters are frequently disposed in the insulation space to maintain vacuum therein after initial mechanical evacuation and sealing of the insulation space. In such manner, the sorbent materials take up the gases which enter the insulation space as a result of leakage through vessel joints and permeation of gases through the walls enclosing the insulation space and evolution of gases within the insulation space resulting from degassing of the materials of construction thereof.
Due to its chemical stability, extremely low gas permeability and low thermal conductivity, glass has been widely used as a material of construction for wall members of double-walled cryogenic liquid containers, particularly those containers of small size commonly referred to as dewar flasks. However, the fragility of glass and its associated susceptibility to breakage has limited its practical utility, particularly with respect to the alternative of using metal as a material of construction for such containers.
Double-walled containers of metal construction are structurally rugged relative to glass and other frangible materials, but are comparatively expensive to fabricate and suffer from the disadvangage that most metals have a relatively high thermal conductivity which in turn results in substantial heat leak from the warm outer wall of the insulation space to the cold inner wall thereof. Despite such shortcomings, double-walled metal containers have been widely used in practice due to the strength, rigidity and low permeability of metals as a material of construction.
More recently, there has been interest in thermoplastic polymeric materials in construction of double-walled cryogenic liquid containers, as a result of the low cost, low thermal conductivity, light weight and ease of forming thereof, as for example by injection molding, rotation molding and the like. Despite the attractiveness of polymeric thermoplastic materials as materials of construction for wall members of cryogenic liquid containers, such materials have the drawback of relatively high gas permeabilities, which has limited the usage of such materials in practice. This is due to the fact that the vacuum which the prior art has sought to continuously maintain in the insulation space of the double-walled container is rapidly degraded unless an excessive amount of gas sorbent material is disposed in the insulation space to take up the substantial volume of gases permeating into the insulation space through the polymeric thermoplastic wall members of the container.
Faced with the foregoing problem of relatively high permeability of polymeric thermoplastic materials of construction, the prior art has proposed various approaches for reduction of permeability of such materials when employed to form vacuum-retaining insulation space wall members. The prior art has for example proposed that the polymeric thermoplastic wall members be provided with a metallic coating, such as by electroplating of the wall surface to be exposed to the vacuum space or by application of metallic foils to such surfaces, so as to reduce permeation inleakage of gas into the insulation space of the container to insignificant levels which can readily be accommodated by small quantities of gas sorbent materials disposed in the insulation space. Another approach of the prior art to reduce permeability of the polymeric thermoplastic wall members has been to provide surface-reacted coatings which chemically bond to selected constituents of the polymeric molecular chains to form a barrier coating on the surface of the thermoplastic polymeric substrate, for reduction of the gas permeability thereof. Yet another approach proposed by the prior art involves the formation of composite insulation space wall members composed of laminates of the polymeric thermoplastic material and a low permeability material such as glass or metal. Although these approaches are to varying degrees successful in reducing permeability of the vacuum-retaining polymeric thermoplastic wall members, such approaches substantially increase the cost and complexity of fabricating the insulation space wall members.
An associated difficulty which has heretofore been encountered in fabrication of double-walled containers, regardless of the materials of construction employed, is the requirement of forming the enclosed insulation space under vacuum conditions so as to avoid the necessity for excessive amounts of gas sorbent materials in the insulation space to achieve at least partial vacuum therein. Accordingly, in conventional practice, double-walled vacuum-insulated containers have either been assembled in a vacuum chamber or else the enclosed insulation space is formed and then evacuated by external means such as vacuum pumps prior to final closure and sealing of the insulation space. The necessity of providing an initial low pressure or vacuum condition in the insulation spaces contributes significantly to the overall cost and complexity of fabricating the double-walled container.
Accordingly, it is an object of the present invention to provide an improved container for cryogenic liquid of the type having spaced-apart inner and outer wall members enclosing a sealed insulation space.
It is another object of the invention to provide an improved method of fabricating a cryogenic liquid container of the above type.
It is another object of the invention to provide a cryogenic liquid container comprising wall members bounding a vacuum insulation space which includes wall member portions formed of high permeability polymeric thermoplastic material uncoated with any permeation barrier coatings.
It is still another object of the invention to provide a method of fabrication of a double-walled vacuum-insulated cryogenic liquid container wherein the container may be completely fabricated under ambient pressure conditions, without the need for any insulation space evacuation means.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.